The associations of leukocyte telomere length and intermediary cardiovascular phenotype with adverse cardiovascular outcomes in the white population

The evidence about the associations of leukocyte telomere length (LTL) and intermediary cardiovascular phenotypes with adverse cardiovascular outcomes is inconclusive. This study assessed these relationships with cardiovascular imaging, electrocardiography, and the risks of sudden cardiac death (SCD), coronary events, and heart failure (HF) admission. We conducted a cross-sectional analysis of UK Biobank participants enrolled between 2006 and 2010. LTL was measured using quantitative polymerase chain reactions. Electronic health records were used to determine the incidence of SCD, coronary events, and HF admission. Cardiovascular measurements were made using cardiovascular magnetic resonance imaging and machine learning. The associations of LTL with SCD, coronary events, and HF admission and cardiac magnetic resonance imaging, electrocardiogram parameters of 33,043 and 19,554 participants were evaluated by multivariate regression. The median (interquartile range) follow-up period was 11.9 (11.2–12.6) years. Data was analyzed from January to May 2023. Among the 403,382 white participants without coronary artery disease or HF, 181,637 (45.0%) were male with a mean age of 57.1 years old. LTL was independently negatively associated with a risk of SCD (LTL third quartile vs first quartile: hazard ratio [HR]: 0.81, 95% confidence interval [CI]: 0.72–0.92), coronary events (LTL third quartile vs first quartile: HR: 0.88, 95% CI: 0.84–0.92), and HF admission (LTL fourth quartile vs first quartile: HR: 0.84, 95% CI: 0.74–0.95). LTL was also independently positively associated with cardiac remodeling, specifically left ventricular mass index, left-ventricular-end systolic and diastolic volumes, mean left ventricular myocardial wall thickness, left ventricular stroke volume, and with electrocardiogram changes along the negative degree of T-axis. Cross-sectional study results showed that LTL was positively associated with heart size and cardiac function in middle age, but electrocardiography results did not show these associations, which could explain the negative association between LTL and risk of SCD, coronary events, and HF admission in UK Biobank participants.


UK Biobank cohort participants
The UK Biobank contains data about 502,421 participants aged 40-69 years old collected from 2006-2010 at 22 assessment centers in England, Scotland, and Wales 12,13 .The UK Biobank contains health-related information about participants that was collected by comprehensive questionnaires, face-to-face interviews, physical measurements, imaging, and blood sampling.Participants were dropped out of the UK Biobank registry if they were not willing to be involved during follow-up.
Of the participants, we excluded 37,871 with extreme LTLs and LTLs shorter than the 1-1.5 interquartile range and longer than 3 + 1.5 interquartile range; 13,480 participants with missing LTLs or white blood cell counts; 1791 participants with implantable cardioverter defibrillators or implanted pacemakers; and 2188 participants with an unknown race or ethnicity 14 .Since there were some reports that leukocyte telomere length is related to ethnicity, and considering most of the participants in the UK Biobank registry are white, we restricted to white race.Among the remaining 447,091 participants, 403,382 white participants with no past medical history of coronary heart disease or HF were included in the study sample (Fig. 1).Of these participants, 33,043 were used for the CMR substudy and 19,554 were used for the ECG substudy.This study received ethical approval from North West-Haydock Research Ethics Committee on June 18, 2021 (REC reference: 21/NM/0157) and was conducted under application No. 77793.Informed consent was obtained from all participants.All of our research methods followed the relevant guidelines and regulations.This study followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guideline for cohort studies (Supplementary Table 1).

Statistical analysis
Participants' baseline characteristics were compared by quartiles of the Z-scores of the log e -transformed adjusted LTL T/S ratio and summarized as mean ± SD for continuous variables and counts and percentages for categorical variables.The time to events was defined as the number of days that passed from the date on which participants enrolled in the UK Biobank registry to the date when a medical event occurred, which was defined as the study endpoint.The primary outcome was SCD incidence.The secondary outcomes were the incidences of coronary events and HF admission.The hazard ratio (HR) and 95% confidence interval (CI) were obtained using multivariable Cox regression analysis adjusting for age, sex, height, weight, hypertension, diabetes, dyslipidemia, and ever smoked history using the lowest quartile of the Z-score of the loge-transformed adjusted LTL T/S ratio as the reference category for both primary and secondary outcomes.Schoenfeld residuals were used to assess proportional hazard assumptions for primary and secondary outcomes.Risk of SCD was additionally adjusted by incident coronary events and HF admission as time-dependent variable.Fine-Grey sub-distribution hazard model was used to perform competing risk multivariable regression, wherein all-cause-mortality was set as the competing risk for SCD.

Ethics declarations
This study received ethical approval from North West-Haydock Research Ethics Committee on June 18, 2021 (REC reference: 21/NM/0157) and was conducted under application No. 77793.Informed consent was obtained from all participants.All of our research methods followed the relevant guidelines and regulations.

Study population
The

Longitudinal association between LTL and SCD
Multivariable Cox regression analysis results for the association between LTL and SCD incidence are presented in Fig. 2 and Supplementary Table 5.During the median follow-up period of 11.9 years (interquartile range: 11.2-12.6years), SCD occurred in 1889 participants with an incidence rate of 0.04 per 100 person-years (PYRs).
Results did not also change in the Fine-Grey competing risk multivariable regression model.Adjusted SCD risk were significantly lower in participants in the second (HR: 0.83, 95% CI: 0.73-0.94,p = 0.002) and third quartiles (HR: 0.82, 95% CI: 0.73-0.93,p = 0.002) than the shortest quartile group.However, the adjusted SCD of the longest quartile was not statistically significantly different from that of the shortest quartile (HR: 0.90, 95% CI: 0.80-1.03,p = 0.127).
Figure 5a shows the association between SCD incidence and LTL using the restricted cubic spline curve.The association between LTL and SCD had a U-shape with a nonlinear p of 0.028.The slope was steeply negative below a standardized Z-score of the loge-adjusted LTL T/S ratio of 0, then reached a plateau and became positive above 0.
After adjusting for clinical variables, there was no association between LTL and ECG parameters, namely QRS duration, QT interval, QTc interval, RR interval, and QRS-axis.However, it was associated with the degree of T-axis (β: − 0.57, 95% CI: − 1.07 to − 0.07, p = 0.027).

Subgroup analysis
Subgroup analysis showed that the incidence ratios for the ≥ 65-years-old and < 65-years old groups were statistically significantly different (p for interaction 0.003) (Table 3).LTL was negatively associated with SCD risk regardless of gender or comorbidities.

Discussion
This study's results showed that LTL (1) was negatively associated with the incidences of SCD, coronary events and HF admission and this association was stronger in those 65 years old or older; (2) independently positively associated with cardiac remodeling, namely LV mass index, LV-end systolic and diastolic volumes, mean LV myocardial wall thickness, and LV stroke volume; and (3) negatively associated with T-axis degree.

LTL association with SCD incidence
This cross-sectional analysis of white participants in the UK Biobank registry showed that LTL was negatively associated with the incidences of SCD, coronary events and HF admission.Participants were 40-69 years old and the two most common causes of SCD in this age group are coronary heart disease and cardiomyopathy, so we chose coronary events and HF admission as the secondary outcomes 23 .Our study showed similar results, which incident coronary event and HF admission were statistically significantly related to SCD.Interestingly, LTL was associated with SCD independent of incident coronary event and HF admission.Compared with the shortest quartile, adjusted SCD risk was significantly lower for the second and third quartiles.Telomere shortening might be related to atherosclerosis through various biological pathways 10 .Other SCD pathophysiology unrelated to atherosclerosis, such as arrhythmogenic disorder in structurally normal hearts, infiltrative heart diseases, and drug abuse, might have caused the longest quartile to have a higher adjusted SCD risk than the third quartile.LTL was negatively associated with SCD because it was negatively associated with incidences of coronary events and HF admission.However, nothing has been clearly revealed about independent associations of LTL with SCD which needs further research.
LTL and SCD were not statistically significantly associated among non-white participants (Supplementary Table 6).The small number of non-white participants may have limited our ability to detect statistically significant differences in the CI and HR for the association between LTL and SCD in non-white participants.

The associations of LTL with heart size and cardiac function
We examined the association between LTL and intermediary cardiovascular phenotypes to determine why LTL was negatively associated with the incidences of SCD coronary events and HF admission.Intermediary cardiovascular phenotypes, such as coronary computed tomography angiography or echocardiography, were not included in the UK Biobank registry.Thus, we examined the association between LTL and other intermediary cardiovascular phenotypes, such as CMR and ECG parameters.LTL was positively associated with cardiac remodeling, including LV mass and LV stroke volume.LV loading is predominantly affected by LV volume, so LV has a similar pathophysiology as athletes' heart 24,25 .Longer LTLs and higher stroke volumes increases positive ventricular loading, leading to higher LV mass and thickness.Similar findings about the association between LTL and LV mass were shown in previous studies 26,27 .This study's CMR results showed that LTL was negatively associated with HF admission incidence in participants without a history of coronary heart disease or HF.
LV strain, myocardial deformation analysis, reflects ischemic damage and wall stress of myocardium.However, in this study, LTL was not associated with LV strain, namely LV circumferential global strain, LV radial www.nature.com/scientificreports/global strain, and LV longitudinal global strain.In the UK Biobank registry, white participants first experienced coronary events onset 11.5 ± 1.9 years after their baseline visit.CMR was obtained 8.8 ± 1.7 years after their baseline visits, so no associations between LTL and LV strain parameters were acceptable, which requires further investigation.
Abnormal T-wave-axes indicate SCD 28 .LTL was negatively associated with SCD incidence, suggesting that LTL is negatively associated with T-wave-axes abnormality.LTL was negatively associated with T-wave-axis.However, considering Z-scores of the log e -transformed adjusted LTL T/S ratio ranges from − 3.2 to 2.2, the β coefficient of − 0.57 was too small to indicate that LTL is associated with T-wave-axis abnormality, so it may not explain SCD incidence.LTL was not associated with any ECG-related parameters, namely QRS duration, QT interval, QTc interval, RR interval, and QRS-axis.

Limitations
This study's first limitation is that it may have been subject to the healthy cohort bias because UK Biobank registry volunteers were more likely to be healthy and have fewer comorbidities than the general UK population.The second limitation is that CMR and ECG parameters were not obtained on the same date as LTL or when coronary events or HF admission occurred.The third limitation is that CMR and ECG were not done for every white participant.Even though we've adjusted age, sex, height, weight, systolic blood pressure, diabetes, dyslipidemia, ever smoked history and total metabolic equivalent of task minutes per week for obtaining associations of LTL with CMR and ECG parameters, participants who were involved in CMR and ECG work-up were more likely to be younger and male and less likely to have ever smoked history and cardiovascular or cerebrovascular comorbidities which can cause selection bias and limits generalizability (Supplementary Table 3 and Supplementary Table 4).The Fourth limitation is that there might be residual confounding variables.Due to observational nature, the current study cannot exclude residual confounding.The fifth limitation is the lack of multiple comparisons testing adjustment in order to adjust the p value related to associations of LTL with CMR and ECG parameters.

Conclusion
This cross-sectional study showed that LTL was positively associated with heart size and better cardiac function in middle-aged participants which could explain the negative association between LTL and risk of SCD, coronary events, and HF admission in the UK Biobank participants.However, ECG results did not show these associations.

Figure 2 .
Figure 2.Longitudinal association between leukocyte telomere length and incidence of sudden cardiac death in white participants.CI confidence interval, HR hazard ratio, LTL leukocyte telomere length, NA not applicable, SCD sudden cardiac death.Cox proportional hazards analysis was adjusted for age, sex, height, weight, hypertension, diabetes, dyslipidemia and ever smoked history.

Figure 3 .
Figure3.Longitudinal association between leukocyte telomere length and incidence of coronary event in white participants.CI confidence interval, HR hazard ratio, LTL leukocyte telomere length, NA not applicable.Coronary event was a composite of death by coronary artery disease or myocardial infarction, any acute myocardial infarction, any coronary revascularization.Cox proportional hazards analysis was adjusted for age, sex, height, weight, hypertension, diabetes, dyslipidemia and ever smoked history.

Figure 4 .
Figure 4. Longitudinal association between leukocyte telomere length and incidence of heart failure admission in white participants.CI confidence interval, HF heart failure, HR hazard ratio, LTL leukocyte telomere length, NA not applicable.Cox proportional hazards analysis was adjusted for age, sex, height, weight, hypertension, diabetes, dyslipidemia and ever smoked history.

Figure 5 .
Figure 5.The dose-response associations of leukocyte telomere length with risk of incident sudden cardiac death (a), coronary event (b) and heart failure admission (c).CI confidence interval, LTL leukocyte telomere length.Restricted cubic spline models were fitted for Cox proportional hazard models, which were adjusted for age, sex, height, weight, hypertension, diabetes, dyslipidemia and ever smoked history.

Table 2 .
The associations of leukocyte telomere length with CMR parameters and ECG parameters in white participants by using linear regression models.CI confidence interval, CMR cardiovascular magnetic resonance, ECG electrocardiogram, LV left ventricle, LVEF left ventricular ejection fraction, LVMI left ventricular mass index.a The analysis was adjusted for age, sex, height, weight, systolic blood pressure, diabetes, dyslipidemia, ever smoked history and total metabolic equivalent of task minutes per week.

Table 3 .
Subgroup analysis results for the association between leukocyte telomere length and sudden cardiac death.CI confidence interval, CKD chronic kidney disease, ESRD end stage renal disease, HR hazard ratio.a Multivariable Cox regression analysis was adjusted for age, sex, height, weight, hypertension, diabetes, dyslipidemia and ever smoked history.Hazard ratio indicates risk of sudden cardiac death per one Z-scores of the log e -transformed adjusted LTL T/S ratio.